Contact Structure of Semiconductor Device

ABSTRACT

The invention relates to a contact structure of a semiconductor device. An exemplary structure for a contact structure for a semiconductor device comprises a substrate comprising a major surface and a trench below the major surface; a strained material filling the trench, wherein a lattice constant of the strained material is different from a lattice constant of the substrate; an inter-layer dielectric (ILD) layer having an opening over the strained material, wherein the opening comprises dielectric sidewalls and a strained material bottom; a semiconductor layer on the sidewalls and bottom of the opening; a dielectric layer on the semiconductor layer; and a metal layer filling an opening of the dielectric layer.

TECHNICAL FIELD

This disclosure relates to integrated circuit fabrication, and moreparticularly to a semiconductor device with a contact structure.

BACKGROUND

As the semiconductor industry has progressed into nanometer technologyprocess nodes in pursuit of higher device density, higher performance,and lower costs, challenges from both fabrication and design issues haveresulted in the development of three-dimensional designs of asemiconductor device, such as a fin field effect transistor (FinFET). Atypical FinFET is fabricated with a thin vertical “fin” (or finstructure) extending from a substrate formed by, for example, etchingaway a portion of a silicon layer of the substrate. The channel of theFinFET is formed in this vertical fin. A gate is provided over threesides (e.g., wrapping) the fin. Having a gate on both sides of thechannel allows gate control of the channel from both sides. Furtheradvantages of FinFET comprise reducing the short channel effect andhigher current flow.

However, there are challenges to implementation of such features andprocesses in complementary metal-oxide-semiconductor (CMOS) fabrication.For example, silicide formation on strained materials causes highcontact resistance of source/drain regions of the FinFET, therebydegrading the device performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 is a flowchart illustrating a method of fabricating a contactstructure of a semiconductor device according to various aspects of thepresent disclosure; and

FIGS. 2-12 are schematic cross-sectional views of a semiconductor devicecomprising a contact structure at various stages of fabricationaccording to various aspects of the present disclosure.

DESCRIPTION

It is understood that the following disclosure provides many differentembodiments, or examples, for implementing different features of thedisclosure. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Forexample, the formation of a first feature over or on a second feature inthe description that follows may include embodiments in which the firstand second features are formed in direct contact, and may also includeembodiments in which additional features may be formed between the firstand second features, such that the first and second features may not bein direct contact. In addition, the present disclosure may repeatreference numerals and/or letters in the various examples. Thisrepetition is for the purpose of simplicity and clarity and does not initself dictate a relationship between the various embodiments and/orconfigurations discussed.

Referring to FIG. 1, illustrated is a flowchart of a method 100 offabricating a contact structure of a semiconductor device according tovarious aspects of the present disclosure. The method 100 begins withstep 102 in which a substrate comprising a major surface and a trenchbelow the major surface is provided. The method 100 continues with step104 in which a strained material is epi-grown in the trench, wherein alattice constant of the strained material is different from a latticeconstant of the substrate. The method 100 continues with step 106 inwhich an inter-layer dielectric (ILD) layer is formed over the strainedmaterial. The method 100 continues with step 108 in which an opening isformed in the ILD layer to expose a portion of the strained material.The method 100 continues with step 110 in which a semiconductor oxidelayer is formed on interior of the opening and extending over the ILDlayer. The method 100 continues with step 112 in which a first metallayer is formed over the semiconductor oxide layer. The method 100continues with step 114 in which the substrate is heated to form asemiconductor layer and a dielectric layer over the semiconductor layer.The method 100 continues with step 116 in which a second metal layer isformed in an opening of the dielectric layer. The discussion thatfollows illustrates embodiments of semiconductor devices that can befabricated according to the method 100 of FIG. 1.

FIGS. 2-12 are schematic cross-sectional views of a semiconductor device200 comprising a contact structure 234 at various stages of fabricationaccording to various aspects of the present disclosure. As employed inthe present disclosure, the term semiconductor device 200 refers to afin field effect transistor (FinFET). The FinFET refers to anyfin-based, multi-gate transistor. In some alternative embodiments, theterm semiconductor device 200 refers to a planarmetal-oxide-semiconductor field effect transistor (MOSFET). Othertransistor structures and analogous structures are within thecontemplated scope of this disclosure. The semiconductor device 200 maybe included in a microprocessor, memory cell, and/or other integratedcircuit (IC).

It is noted that, in some embodiments, the performance of the operationsmentioned in FIG. 1 does not produce a completed semiconductor device200. A completed semiconductor device 200 may be fabricated usingcomplementary metal-oxide-semiconductor (CMOS) technology processing.Accordingly, it is understood that additional processes may be providedbefore, during, and/or after the method 100 of FIG. 1, and that someother processes may only be briefly described herein. Also, FIGS. 2through 12 are simplified for a better understanding of the concepts ofthe present disclosure. For example, although the figures illustrate thesemiconductor device 200, it is understood the IC may comprise a numberof other devices comprising resistors, capacitors, inductors, fuses,etc.

Referring to FIG. 2 and step 102 in FIG. 1, a substrate 20 comprising amajor surface 20 s is provided. In at least one embodiment, thesubstrate 20 comprises a crystalline silicon substrate (e.g., wafer).The substrate 20 may comprise various doped regions depending on designrequirements (e.g., p-type substrate or n-type substrate). In someembodiments, the doped regions may be doped with p-type or n-typedopants. For example, the doped regions may be doped with p-typedopants, such as boron or BF₂; n-type dopants, such as phosphorus orarsenic; and/or combinations thereof. The doped regions may beconfigured for an n-type FinFET or planar MOSFET, or alternativelyconfigured for a p-type FinFET or planar MOSFET.

The substrate 20 may alternatively be made of some other suitableelementary semiconductor, such as diamond or germanium; a suitablecompound semiconductor, such as gallium arsenide, silicon carbide,indium arsenide, or indium phosphide; or a suitable alloy semiconductor,such as silicon germanium carbide, gallium arsenic phosphide, or galliumindium phosphide. Further, the substrate 20 may include an epitaxiallayer (epi-layer), may be strained for performance enhancement, and/ormay include a silicon-on-insulator (SOI) structure.

In the depicted embodiment, the substrate 20 further comprises a finstructure 202. The fin structure 202, formed on the substrate 20,comprises one or more fins. In the present embodiment, for simplicity,the fin structure 202 comprises a single fin. The fin comprises anysuitable material, for example, the fin may comprise silicon, germaniumor compound semiconductor. The fin structure 202 may further comprise acapping layer (not shown) disposed on the fin, which may be asilicon-capping layer.

The fin structure 202 is formed using any suitable process comprisingvarious deposition, photolithography, and/or etching processes. Anexemplary photolithography process may include forming a photoresistlayer (resist) overlying the substrate 20 (e.g., on a silicon layer),exposing the resist to a pattern, performing a post-exposure bakeprocess, and developing the resist to form a masking element includingthe resist. The silicon layer may then be etched using reactive ionetching (RIE) processes and/or other suitable processes. In an example,silicon fins of the fin structure 202 may be formed using patterning andetching a portion of the silicon substrate 20. In another example,silicon fins of the fin structure 202 may be formed using patterning andetching a silicon layer deposited overlying an insulator layer (forexample, an upper silicon layer of a silicon-insulator-silicon stack ofan SOI substrate). In still other embodiments, the fin structure isformed by forming a dielectric layer above a substrate, opening trenchesin the dielectric layer, and epitaxially growing fins from the substratein the trenches to form the fins.

In the depicted embodiment, isolation regions are formed within thesubstrate 20 to define and electrically isolate the various fins of thefin structure 202. In one example, the isolation regions include shallowtrench isolation (STI) regions 204 (comprising 204 a and 204 b). Theisolation regions may comprise silicon oxide, silicon nitride, siliconoxynitride, fluoride-doped silicate glass (FSG), a low-K dielectricmaterial, and/or combinations thereof. The isolation regions, and in thepresent embodiment, the STI regions 204, may be formed by any suitableprocess. As one example, the formation of the STI regions 204 mayinclude filling trenches between the fins (for example, using a chemicalvapor deposition process) with a dielectric material. In someembodiments, the filled trench may have a multi-layer structure such asa thermal oxide liner layer filled with silicon nitride or siliconoxide.

Still referring to FIG. 2, a gate stack 210 is formed on the majorsurface 20 s of the substrate 20 (i.e., a top surface of the finstructure 202) in between the STI regions 204. Although in the planeillustrated in the Figures, gate stack 210 extends only on the topsurface of the fin, those skilled in the art will recognize that inanother plane of the device (not shown in the drawings) gate stack 210extends along the sidewalls of fin structure 202. In some embodiments,the gate stack 210 comprises a gate dielectric layer 212 and a gateelectrode layer 214 over the gate dielectric layer 212. In someembodiments, a pair of sidewall spacers 216 is formed on two sides ofthe gate stack 210. In the depicted embodiment, the gate stack 210 maybe formed using any suitable process, including the processes describedherein.

In one example, the gate dielectric layer 212 and gate electrode layer214 are sequentially deposited over the substrate 20. In someembodiments, the gate dielectric layer 212 may include silicon oxide,silicon nitride, silicon oxy-nitride, or high dielectric constant(high-k) dielectric. High-k dielectrics comprise metal oxides. Examplesof metal oxides used for high-k dielectrics include oxides of Li, Be,Mg, Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, Lu and mixtures thereof. In the present embodiment, the gatedielectric layer 212 is a high-k dielectric layer with a thickness inthe range of about 10 angstroms to about 30 angstroms. The gatedielectric layer 212 may be formed using a suitable process such asatomic layer deposition (ALD), chemical vapor deposition (CVD), physicalvapor deposition (PVD), thermal oxidation, UV-ozone oxidation, orcombinations thereof. The gate dielectric layer 212 may further comprisean interfacial layer (not shown) to reduce damage between the gatedielectric layer 212 and the fin structure 202. The interfacial layermay comprise silicon oxide.

In some embodiments, the gate electrode layer 214 may comprise asingle-layer or multilayer structure. In at least one embodiment, thegate electrode layer 214 comprises poly-silicon. Further, the gateelectrode layer 214 may be doped poly-silicon with the uniform ornon-uniform doping. In an alternative embodiment, the gate electrodelayer 214 comprises a metal selected from a group of W, Cu, Ti, Ag, Al,TiAl, TiAlN, TaC, TaCN, TaSiN, Mn, and Zr. In an alternative embodiment,the gate electrode layer 214 comprises a metal selected from a group ofTiN, WN, TaN, and Ru. In the present embodiment, the gate electrodelayer 214 comprises a thickness in the range of about 30 nm to about 60nm. The gate electrode layer 214 may be formed using a suitable processsuch as ALD, CVD, PVD, plating, or combinations thereof.

Then, a layer of photoresist (not shown) is formed over the gateelectrode layer 214 by a suitable process, such as spin-on coating, andpatterned to form a patterned photoresist feature by a properlithography patterning method. In at least one embodiment, a width ofthe patterned photoresist feature is in the range of about 5 nm to about45 nm. The patterned photoresist feature can then be transferred using adry etching process to the underlying layers (i.e., the gate electrodelayer 214 and the gate dielectric layer 212) to form the gate stack 210.The photoresist layer may be stripped thereafter.

Still referring to FIG. 2, the semiconductor device 200 furthercomprises a dielectric layer formed over the gate stack 210 and thesubstrate 20 and covering sidewalls of the gate stack 210. Thedielectric layer may include silicon oxide, silicon nitride, or siliconoxy-nitride. The dielectric layer may comprise a single layer ormultilayer structure. The dielectric layer may be formed by CVD, PVD,ALD, or other suitable technique. The dielectric layer comprises athickness ranging from about 5 nm to about 15 nm. Then, an anisotropicetching is performed on the dielectric layer to form the pair ofsidewall spacers 216 on two sides of the gate stack 210.

Referring to FIG. 3 and step 102 in FIG. 1, portions of the finstructure 202 (other than where the gate stack 210 and the pair ofsidewall spacers 216 are formed thereover) are recessed to form sourceand drain (S/D) trenches 206 (comprising 206 a and 206 b) below themajor surface 20 s of the substrate 20 adjacent to the gate stack 210.In the depicted embodiment, each of the S/D trenches 206 is between thegate stack 210 and one of the STI regions 204. As such, the S/D trench206 a is adjacent to the gate stack 210, while the STI region 204 a isdisposed on a side of the S/D trench 206 a opposite the gate stack 210.As such, the S/D trench 206 b is adjacent to the gate stack 210, whilethe STI region 204 b is disposed on a side of the S/D trench 206 bopposite the gate stack 210.

In the depicted embodiment, using the gate stack 210 and the pair ofsidewall spacers 216 as hard masks, a biased etching process isperformed to recess the major surface 20 s of the substrate 20 that areunprotected or exposed to form the S/D trenches 206. In one embodiment,the etching process may be performed under a pressure of about 1 mTorrto about 1000 mTorr, a power of about 50 W to about 1000 W, a biasvoltage of about 20 V to about 500 V, at a temperature of about 40° C.to about 60° C., using a HBr and/or Cl₂ as etch gases. Also, in theembodiments provided, the bias voltage used in the etching process maybe tuned to allow better control of an etching direction to achievedesired profiles for the S/D trenches 206.

As depicted in FIG. 4 and step 104 in FIG. 1, after the formation of theS/D trenches 206 below the major surface 20 s of the substrate 20, thestructure in FIG. 4 is produced by epi-growing a strained material 208in the S/D trench 206, wherein a lattice constant of the strainedmaterial 208 is different from a lattice constant of the substrate 20.Thus, the channel region of the semiconductor device 200 is strained orstressed to enhance carrier mobility of the device.

In some embodiments, the strained material 208 comprises Si, Ge, SiGe,SiC, SiP, or III-V semiconductor material. In the depicted embodiment, apre-cleaning process may be performed to clean the S/D trenches 206 withHF or other suitable solution. Then, the strained material 208 such assilicon germanium (SiGe) is selectively grown by a low-pressure CVD(LPCVD) process to fill the S/D trenches 206. In one embodiment, anupper surface of the strained material 208 is lower than the majorsurface 20 s (not shown). In another embodiment, the strained material208 filling the S/D trenches 206 extends upward over the major surface20 s. In the depicted embodiment, the LPCVD process is performed at atemperature of about 400 to about 800° C. and under a pressure of about1 to about 15 Ton, using SiH₂Cl₂, HCl, GeH₄, B₂H₆, and H₂ as reactiongases.

The process steps up to this point have provided the substrate 20 havingthe strained material 208 in the S/D trenches 206. In some applications,silicide regions over the strained material 208 may be formed by blanketdepositing a thin layer of metal material, such as nickel, titanium,cobalt, and combinations thereof. The substrate 20 is then heated, whichcauses silicon to react with the metal where contacted. After thereaction, a layer of metal silicide is formed between thesilicon-containing material and the metal. The un-reacted metal isselectively removed through the use of an etchant that attacks the metalmaterial but does not attack silicide. However, Fermi level pinningbetween the metal silicide and strained material 208 results in a fixedSchottky barrier height (SBH). This fixed SBH causes high contactresistance of S/D regions of the semiconductor device and thus degradesthe device performance.

Accordingly, the processing discussed below with reference to FIGS. 5-12may form a contact structure comprising a conductive dielectric layer toreplace the silicide regions. The conductive dielectric layer may serveas a low-resistance intermediate layer to replace high-resistance metalsilicide. As such, the contact structure may provide low contactresistance of S/D regions of the semiconductor device, thereby enhancingthe device performance.

As depicted in FIGS. 5 and 6 and step 106 in FIG. 1, for fabricating acontact structure (such as a contact structure 234 shown in FIG. 12) ofthe semiconductor device 200, the structure in FIG. 5 is produced byforming an inter-layer dielectric (ILD) layer 218 over the strainedmaterial 208, the gate stack 210, the pair of sidewall spacers 216 andthe isolation regions 204.

The ILD layer 218 comprises a dielectric material. The dielectricmaterial may comprise silicon oxide, silicon nitride, siliconoxynitride, phosphosilicate glass (PSG), borophosphosilicate glass(BPSG), spin-on glass (SOG), fluorinated silica glass (FSG), carbondoped silicon oxide (e.g., SiCOH), and/or combinations thereof. In someembodiments, the ILD layer 218 may be formed over the strained material208 by CVD, high density plasma (HDP) CVD, sub-atmospheric CVD (SACVD),spin-on, sputtering, or other suitable methods. In the presentembodiment, the ILD layer 218 has a thickness in the range of about 4000Å to about 8000 Å. It is understood that the ILD layer 218 may compriseone or more dielectric materials and/or one or more dielectric layers.

Subsequently, the ILD layer 218 is planarized using a CMP process untila top surface of the gate electrode layer 214 is exposed or reached(shown in FIG. 6). The CMP process has a high selectivity to provide asubstantially planar surface for the gate electrode layer 214 and ILDlayer 218.

Subsequent CMOS processing steps applied to the semiconductor device 200of FIG. 6 comprise forming contact opening through the ILD layer 218 toprovide electrical contacts to S/D regions of the semiconductor device200. Referring to FIG. 7, the structure in FIG. 7 is produced by formingan opening 220 in the ILD layer 218 to expose a portion of the strainedmaterial 208 (step 108 in FIG. 1). As one example, the formation of theopening 220 includes forming a layer of photoresist (not shown) over theILD layer 218 by a suitable process, such as spin-on coating, patterningthe layer of photoresist to form a patterned photoresist feature by aproper lithography method, etching the exposed ILD layer 218 (forexample, by using a dry etching, wet etching, and/or plasma etchingprocess) to remove portions of the ILD layer 218 to expose a portion ofthe strained material 208. As such, the opening 220 is over the strainedmaterial 208, wherein the opening 220 comprises dielectric sidewalls 220a and a strained material bottom 220 b. The patterned photoresist layermay be stripped thereafter.

Referring to FIG. 8 and step 110 in FIG. 1, after formation of theopening 220 in the ILD layer 218, the structure in FIG. 8 is produced byforming a semiconductor oxide layer 222 on interior of the opening 220and extending over the ILD layer 218 and the gate stack 210. In someembodiments, the semiconductor oxide layer 222 may comprise siliconoxide or germanium oxide, and may be formed using a method such as CVD,ALD or sputtering. In some embodiments, the semiconductor oxide layer222 has a first thickness t₁ ranging from about 0.6 nm to about 3 nm.

Referring to FIG. 9 and step 112 in FIG. 1, following formation of thesemiconductor oxide layer 222 on interior of the opening 220, thestructure in FIG. 9 is produced by forming a first metal layer 224 overthe semiconductor oxide layer 222. In some embodiments, the first metallayer 224 may comprise Ti, Al, Zr, Hf, Ta, In, Ni, Be, Mg, Ca, Y, Ba,Sr, Sc, or Ga, and may be formed using a method such as CVD, ALD orsputtering. In some embodiments, the first metal layer 224 has a secondthickness t₂ ranging from about 0.5 nm to about 4 nm.

Referring to FIG. 10 and step 114 in FIG. 1, subsequent to the formationof the first metal layer 224 over the semiconductor oxide layer 222, thestructures in FIG. 10 is produced by heating the substrate 20 to form asemiconductor layer 226 and a dielectric layer 228 over thesemiconductor layer 226. In some embodiments, the semiconductor layer226 comprises Si or Ge. In some embodiments, the semiconductor layer 226has a third thickness t₃ ranging from 0.3 nm to 1.5 nm. In someembodiments, the dielectric layer 228 partially filling the opening 220has an opening 230. In some embodiments, the dielectric layer 228 has afourth thickness t₄ ranging from about 1 nm to about 10 nm, making thedielectric layer 228 conductive. While the teaching of this disclosureis not limited to a specific theory of operation, it is believed that atthe disclosed thickness range, dielectric layer 228 is a conductivedielectric layer because of tunneling current. As such, the dielectriclayer 228 is referred to as a conductive dielectric layer 228 hereafter.In at least one embodiment, the conductive dielectric layer 228comprises TiO or TiO₂. In an alternative embodiment, the conductivedielectric layer 228 comprises Al₂O₃. In an alternative embodiment, theconductive dielectric layer 228 is selected from an oxide of the groupconsisting of Zr, Hf, Ta, In, Ni, Be, Mg, Ca, Y, Ba, Sr, Sc, Ga, andmixtures thereof. In the depicted embodiment, the conductive dielectriclayer 228 may reduce the fixed SBH and serve as a low-resistanceintermediate layer to replace high-resistance metal silicide, therebyenhancing the device performance.

Thermodynamically, the semiconductor layer 226 is more stable than thefirst metal layer 224 in an oxygen-containing environment. As such, thefirst metal layer 224 may convert the semiconductor oxide layer 222 incontact therewith to form the semiconductor layer 226, while the firstmetal layer 224 is oxidized to form the conductive dielectric layer 228over the semiconductor layer 226. In the depicted embodiment, thesemiconductor layer 226 is on the sidewalls 220 a and bottom 220 b ofthe opening 220. In some embodiments, the step of heating the substrate20 is performed by exposing the substrate 20 to an inert gas, at atemperature of about 200° C. to about 800° C. In some embodiments, theinert gas comprises N₂, He, or Ar.

Referring to FIGS. 11 and 12 and step 116 in FIG. 1, following formationof the conductive dielectric layer 228, the structures in FIG. 11 isproduced by forming a second metal layer 232 in the opening 230 of thedielectric layer 228. In the depicted embodiment, the second metal layer232 is deposited over the conductive dielectric layer 228 to fill theopening 230 of the conductive dielectric layer 228. In some embodiments,the second metal layer 232 comprises Ta, Ti, Hf, Zr, Ni, W, Co, Cu, orAl. In some embodiments, the second metal layer 232 may be formed byCVD, PVD, plating, ALD, or other suitable technique. In some embodiment,the second metal layer 232 may comprise a laminate. The laminate mayfurther comprise a barrier metal layer, a liner metal layer or a wettingmetal layer. Further, the thickness of the second metal layer 232 willdepend on the depth of the opening 230. The second metal layer 232 isthus deposited until the opening 230 are substantially filled orover-filled.

Then, another CMP is performed to planarize the second metal layer 232after filling the opening 230 (shown in FIG. 12). Since the CMP removesa portion of the second metal layer 232 outside of the opening 230, theCMP process may stop when reaching the ILD layer 218, and thus providinga substantially planar surface.

In some embodiments, with respect to the example depicted in FIG. 12,the contact structure 234 for the semiconductor device 200 comprises thesubstrate 20 comprising the major surface 20 s and the trench 206 belowthe major surface 20 s; the strained material 208 filling the trench206, wherein a lattice constant of the strained material 208 isdifferent from a lattice constant of the substrate 20; the ILD layer 218having the opening 220 over the strained material 208, wherein theopening 220 comprises the dielectric sidewalls 220 a and the strainedmaterial bottom 220 b; the semiconductor layer 226 on the sidewalls 220a and bottom 220 b of the opening 220; the dielectric layer 228 on thesemiconductor layer 226; and the metal layer 232 filling the opening 230of the dielectric layer 228.

In the depicted embodiment, the gate stack 210 is fabricated using agate-first process. In an alternative embodiment, the gate stack 210 maybe fabricated using a gate-last process performed by first forming adummy gate stack. In some embodiments, the gate-last process comprisesforming an ILD layer surrounding the dummy gate stack, removing a dummygate electrode layer to form a trench in the ILD layer, then filling thetrench with a conductive gate electrode layer. In some embodiments, thegate-last process comprises forming an ILD layer surrounding the dummygate stack, removing a dummy gate electrode layer and a dummy gatedielectric layer to form a trench in the ILD layer, then filling thetrench with a gate dielectric layer and a conductive gate electrodelayer.

After the steps shown in FIG. 1, as further illustrated with respect tothe example depicted in FIGS. 2-12, have been performed, subsequentprocesses, comprising interconnect processing, are performed to completethe semiconductor device 200 fabrication. It has been observed that thecontact structure 234 comprising a conductive dielectric layer 228 mayprovide a low-resistance path for interconnection, thus upgrading thedevice performance.

In accordance with embodiments, a contact structure for a semiconductordevice comprises a substrate comprising a major surface and a trenchbelow the major surface; a strained material filling the trench, whereina lattice constant of the strained material is different from a latticeconstant of the substrate; an inter-layer dielectric (ILD) layer havingan opening over the strained material, wherein the opening comprisesdielectric sidewalls and a strained material bottom; a semiconductorlayer on the sidewalls and bottom of the opening; a dielectric layer onthe semiconductor layer; and a metal layer filling an opening of thedielectric layer.

In accordance with another embodiments, a metal oxide semiconductorfield effect transistor (MOSFET) comprises a substrate comprising amajor surface; a gate stack on the major surface of the substrate; atrench below the major surface adjacent to the gate stack; a shallowtrench isolations (STI) region disposed on a side of the trench oppositethe gate stack, wherein the STI region is within the substrate; and acontact structure comprising a strained material filling the trench,wherein a lattice constant of the strained material is different from alattice constant of the substrate; an inter-layer dielectric (ILD) layerhaving an opening over the strained material, wherein the openingcomprises dielectric sidewalls and a strained material bottom; asemiconductor layer on the sidewalls and bottom of the opening, whereinthe semiconductor layer has a thickness ranging from 0.3 nm to 1.5 nm; adielectric layer on the semiconductor layer, wherein the dielectriclayer has a thickness ranging from 1 nm to 10 nm; and a metal layerfilling an opening of the dielectric layer.

In accordance with another embodiments, a method of fabricating asemiconductor device comprises providing a substrate comprising a majorsurface and a trench below the major surface; epi-growing a strainedmaterial in the trench, wherein a lattice constant of the strainedmaterial is different from a lattice constant of the substrate; formingan inter-layer dielectric (ILD) layer over the strained material;forming an opening in the ILD layer to expose a portion of the strainedmaterial; forming a semiconductor oxide layer on interior of the openingand extending over the ILD layer; forming a first metal layer over thesemiconductor oxide layer; heating the substrate to form a semiconductorlayer and a dielectric layer over the semiconductor layer; and forming asecond metal layer in an opening of the dielectric layer.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

What is claimed is:
 1. A contact structure for a semiconductor devicecomprising: a substrate comprising a major surface and a trench belowthe major surface; a strained material filling the trench, wherein alattice constant of the strained material is different from a latticeconstant of the substrate; an inter-layer dielectric (ILD) layer havingan opening over the strained material, wherein the opening comprisesdielectric sidewalls and a strained material bottom; a semiconductorlayer on the sidewalls and bottom of the opening; a dielectric layer onthe semiconductor layer; and a metal layer filling an opening of thedielectric layer.
 2. The contact structure of claim 1, wherein thesemiconductor layer has a thickness ranging from 0.3 nm to 1.5 nm. 3.The contact structure of claim 1, wherein the dielectric layer has athickness ranging from 1 nm to 10 nm.
 4. The contact structure of claim1, wherein the strained material comprises Si, Ge, SiGe, SiC, SiP, orIII-V semiconductor material.
 5. The contact structure of claim 1,wherein the semiconductor layer comprises Si or Ge.
 6. The contactstructure of claim 1, wherein the dielectric layer comprises TiO orTiO₂.
 7. The contact structure of claim 1, wherein the dielectric layercomprises Al₂O₃.
 8. The contact structure of claim 1, wherein thedielectric layer is selected from an oxide of the group consisting ofZr, Hf, Ta, In, Ni, Be, Mg, Ca, Y, Ba, Sr, Sc, Ga, and mixtures thereof.9. The contact structure of claim 1, wherein the metal layer comprisesTa, Ti, Hf, Zr, Ni, W, Co, Cu, or Al.
 10. A metal oxide semiconductorfield effect transistor (MOSFET) comprising: a substrate comprising amajor surface; a gate stack on the major surface of the substrate; atrench below the major surface adjacent to the gate stack; a shallowtrench isolations (STI) region disposed on a side of the trench oppositethe gate stack, wherein the STI region is within the substrate; and acontact structure comprising a strained material filling the trench,wherein a lattice constant of the strained material is different from alattice constant of the substrate; an inter-layer dielectric (ILD) layerhaving an opening over the strained material, wherein the openingcomprises dielectric sidewalls and a strained material bottom; asemiconductor layer on the sidewalls and bottom of the opening, whereinthe semiconductor layer has a thickness ranging from 0.3 nm to 1.5 nm; adielectric layer on the semiconductor layer, wherein the dielectriclayer has a thickness ranging from 1 nm to 10 nm; and a metal layerfilling an opening of the dielectric layer.
 11. The MOSFET of claim 10,wherein the strained material comprises Si, Ge, SiGe, SiC, SiP, or III-Vsemiconductor material.
 12. The MOSFET of claim 10, wherein thesemiconductor layer comprises Si or Ge.
 13. The MOSFET of claim 10,wherein the dielectric layer comprises TiO or TiO₂.
 14. The MOSFET ofclaim 10, wherein the dielectric layer comprises Al₂O₃.
 15. The MOSFETof claim 10, wherein the dielectric layer is selected from an oxide ofthe group consisting of Zr, Hf, Ta, In, Ni, Be, Mg, Ca, Y, Ba, Sr, Sc,Ga, and mixtures thereof.
 16. The MOSFET of claim 10, wherein the metallayer comprises Ta, Ti, Hf, Zr, Ni, W, Co, Cu, or Al.
 17. A method offabricating a semiconductor device, comprising: providing a substratecomprising a major surface and a trench below the major surface;epi-growing a strained material in the trench, wherein a latticeconstant of the strained material is different from a lattice constantof the substrate; forming an inter-layer dielectric (ILD) layer over thestrained material; forming an opening in the ILD layer to expose aportion of the strained material; forming a semiconductor oxide layer oninterior of the opening and extending over the ILD layer; forming afirst metal layer over the semiconductor oxide layer; heating thesubstrate to form a semiconductor layer and a dielectric layer over thesemiconductor layer; and forming a second metal layer in an opening ofthe dielectric layer.
 18. The method of claim 17, wherein the step ofheating the substrate to form a semiconductor layer and a dielectriclayer over the semiconductor layer is performed by exposing thesubstrate to an inert gas.
 19. The method of claim 18, wherein the inertgas comprises N₂, He, or Ar.
 20. The method of claim 18, wherein thestep of exposing the substrate to an inert gas is performed at atemperature of about 200° C. to about 800° C.